1. Field of the Invention
This invention relates generally to devices that emit electromagnetic radiation and, in particular, to tunable vertical cavity surface emitting lasers (VCSELs).
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Lasers have a wide range of industrial and scientific uses. There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, and free electron lasers. Semiconductor lasers are also in use. In semiconductor lasers, electromagnetic waves are amplified in a semiconductor superlattice structure. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers are used for a variety of applications and can be built with different structures and semiconductor materials, such as gallium arsenide (GaAs).
The use of semiconductor lasers for forming a source of optical energy is attractive for a number of reasons. Semiconductor lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, semiconductor lasers can be fabricated as monolithic devices, which do not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
A semiconductor laser typically comprises an active (optical gain) region sandwiched between two mirrors (reflectors or reflective planes). There is typically a small difference in reflectivity between the two mirrors, one of which (typically, the reflective plane having lower reflectivity) serves as the xe2x80x9cexitxe2x80x9d mirror. The area between the reflective planes is often referred to as the resonator, or the Fabry-Perot resonance cavity in some cases. The active region is located within the resonant cavity. When the active region is pumped with an appropriate pumping energy, it produces photons, some of which resonate and build up to form coherent light in the resonant cavity formed by the two mirrors. A portion of the coherent light built up in the resonating cavity formed by the active region and top and bottom mirrors passes through one of the mirrors (the exit mirror) as the output laser beam.
Various forms of pumping energy may be utilized to cause the active region to begin to emit photons and to achieve gain. For example, semiconductor lasers of various types may be electrically pumped (EP) (by a DC or alternating current), or pumped in other ways, such as by optical pumping (OP) or electron beam pumping. In an EP VCSEL, for example, an electrical potential difference is typically applied across the active region (via top and bottom electrical contacts provided above and below the active region). As a result of the potential applied, a pumping current flows through the active region, i.e. charge carriers (electrons and holes) are injected from opposite directions into the active region where recombination of electron and holes occurs. There are two kinds of recombination events, i.e. radiative and non-radiative, concurrently happening in the active region. When radiative recombination occurs, a photon is emitted with the same energy as the difference in energy between the hole and electron energy states. Some of those photons travel in a direction perpendicular to the reflectors of the laser. As a result of the ensuing reflections, the photons can travel through the active region multiple times.
Stimulated emission occurs when radiative recombination of an electron-hole pair is stimulated by interaction with a photon. In particular, stimulated emission occurs when a photon with an energy equal to the difference between an electron""s energy and a lower energy interacts with the electron. In this case, the photon stimulates the electron to fall into the lower energy state, thereby emitting a second photon. The second photon will have the same energy and frequency as the original photon, and will also be in phase with the original photon. Thus, when the photons produced by spontaneous electron transition interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. (Viewed as waves, the atom emits a wave having twice the amplitude as that of the original photon interacting with the atom.) If a sufficient amount of radiative recombinations are stimulated by photons, the number of photons traveling between the reflectors tends to increase, giving rise to amplification of light and lasing. The result is that coherent light builds up in the resonant cavity formed by the two mirrors, a portion of which passes through the exit mirror as the output laser beam.
Semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, while in SELs, the radiation output is perpendicular to the wafer surface. One type of SEL is the vertical-cavity surface-emitting laser (VCSEL). The xe2x80x9cverticalxe2x80x9d direction in a VCSEL is the direction perpendicular to the plane of the substrate on which the constituent layers are deposited or epitaxially grown, with xe2x80x9cupxe2x80x9d being typically defined as the direction of epitaxial growth. In some designs, the output laser beam is emitted out of the top side, in which case the top mirror is the exit mirror. In other designs, the laser beam is emitted from the bottom side, in which case the bottom mirror is the exit mirror. The exit mirror typically has slightly lower reflectivity than the other mirror.
VCSELs have many attractive features compared to edge-emitting lasers, such as low threshold current, single longitudinal mode, a circular output beam profile, a smaller divergence angle, and scalability to monolithic laser arrays. The shorter cavity resonator of the VCSEL provides for better longitudinal mode selectivity, and hence narrower linewidths. Additionally, because the output is perpendicular to the wafer surface, it is possible to test fabricated VCSELs on the wafer before extensive packaging is done, in contrast to edge-emitting lasers, which must be cut from the wafer to test the laser. Also, because the cavity resonator of the VCSEL is perpendicular to the layers, there is no need for the cleaving operation common to edge-emitting lasers.
The VCSEL structure usually consists of an active (optical gain) region sandwiched between two mirrors, such as distributed Bragg reflector (DBR) mirrors. Both EP and OP VCSEL designs are possible. The two mirrors may be referred to as a top DBR and a bottom DBR. Because the optical gain is low in a vertical cavity design, the reflectors require a high reflectivity in order to achieve a sufficient level of feedback for the device to lase.
DBRs are typically formed of multiple pairs of layers referred to as mirror pairs. DBRs are sometimes referred to as mirror stacks. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL, to permit epitaxial fabrication techniques. The layers of the DBR are quarter-wave optical-thickness (QWOT) layers of alternating high and low refractive indices, where each mirror pair contains one high and one low refractive index QWOT layer. The number of mirror pairs per stack may range from 20-40 pairs to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. A larger number of mirror pairs increases the percentage of reflected light (reflectivity).
The DBR mirrors of a typical VCSEL can be constructed from dielectric (insulating) or semiconductor layers (or a combination of both, including metal mirror sections). The difference between the refractive indices of the layers of the mirror pairs can be higher in dielectric DBRs, generally imparting higher reflectivity to dielectric DBRs than to semiconductor DBRs for the same number of mirror pairs and overall thickness. Conversely, in a dielectric DBR, a smaller number of mirror pairs can achieve the same reflectivity as a larger number in a semiconductor DBR. However, it is sometimes necessary or desirable to use semiconductor DBRs, despite their lower reflectivity/greater thickness, to conduct current, for example (e.g., in an EP VCSEL). Semiconductor DBRs also have higher thermal (heat) conductivity than do dielectric DBRs, making them more desirable for heat-removal purposes, other things being equal. Semiconductor DBRs may also be preferred for manufacturing reasons (e.g., a thicker DBR may be needed for support) or fabrication reasons (e.g., an epitaxial, i.e. semiconductor, DBR may be needed if other epitaxial layers need to be grown on top of the DBR).
When properly designed, these mirror pairs will cause a desired reflectivity at the laser wavelength. Typically in a VCSEL, the mirrors are designed so that the bottom DBR mirror (i.e. the one interposed between the substrate material and the active region) has nearly 100% reflectivity, while the top (exit) DBR mirror has a reflectivity that may be 98%-99.5% (depending on the details of the laser design). The partially reflective top (exit) mirror passes a portion of the coherent light built up in the resonating cavity formed by the active region and top and bottom mirrors. Of course, as noted above, in other designs, the bottom mirror may serve as the exit mirror and the top mirror have the higher reflectivity. VCSELs, DBRs, and related matters are discussed in further detail in Vertical-Cavity Surface-Emitting Lasers: Design, Fabrication, Characterization, and Applications, eds. Carl W. Wilmsen, Henryk Temkin and Larry A. Coldren (Cambridge: Cambridge University Press, 1999).
VCSELs are used in a variety of applications. In telecommunications, for example, output laser light of a precise wavelength is modulated to encode and transmit information. The laser may be externally modulated, or directly modulated. A typical telecommunications system uses optical fiber to guide the radiation from the modulation (or emission) point to the detection point. Long wavelength (1.3 xcexcm to 1.55 xcexcm) VCSELs are of great interest in the optical telecommunications industry because of the minimum fiber dispersion at 1310 nm and the minimum fiber loss at 1.55 xcexcm (1550 nm).
In various communicationsxe2x80x94e.g., telecommunicationsxe2x80x94applications, it is desirable that the emitted laser radiation of a given semiconductor laser have one of a number of precisely specified wavelengths, for example those specified by the International Telecommunications Union (ITU) grid. For example, the ITU grid specifies lasing wavelengths of 1.55 xcexcm (and other closely spaced wavelengths). Other lasing wavelengths, such as 1310 nm (and other closely spaced wavelengths), may also be employed. The ITU grid wavelengths and others are used in telecommunications applications such as coarse and dense wavelength-division multiplexing (CWDM and DWDM). In WDM, typically used in optical fiber communications, two or more optical (e.g. laser) signals having different wavelengths are simultaneously transmitted in the same direction over one fiber, and then are separated by wavelength at the distant end. In the act event of a failure of a laser operating at a particular wavelength, for example, or a change in the particular wavelengths needed for a particular link, reconfiguration of the wavelength at which a laser can operate would be extremely useful.
For at least these reasons, it is desirable to provide a tunable VCSEL, i.e. a VCSEL having an output wavelength corresponding to a selectable tuning parameter. Some approaches used in attempts to tune various types of lasers are described in B. Pezeshki, xe2x80x9cNew Approaches to Laser Tuning,xe2x80x9d Optics and Photonics News, 34-38 (May 2001). These include temperature variation, combination of multiple lasers having different wavelengths on a single chip, and movement of micromechanical components. The wavelength range provided by temperature variation is relatively small, however, and the other approaches may be undesirably complex. Tuning of a laser by direct application of an electrical signal would therefore be desirable. But such tuning should preferably be independent of other properties of the laser. For example, VCSELs can have a wavelength significantly dependent on drive current (or some other tuning parameter). However, it is typically also desired that the gain (i.e., output power or intensity) be constant (or otherwise selectable) over different wavelengths. Unfortunately, when the drive current is changed in order to change wavelength, in conventional VCSELs the gain is also changed, since it is also related to the drive current.
There is, therefore, a need for improved VCSELs and methods for tuning same while controlling their gain.